Helmet mounted displays have been developed for diverse applications, such as simulation systems used for training, and for use in vehicular or aircraft operation as displays for sensor data and/or instrument readings, or as an aid to the operator of the vehicle or aircraft performing other tasks. For example, a helmet mounted display has been used to present data concerning weapon status to pilots of fighter aircraft. This data is often presented in alpha-numeric form and appears to reside or originate far in front of the aircraft but is actually generated by a display, often by a cathode ray tube (CRT), located in the users helmet. Special optics which are attached to the helmet bend and focus the display light to create the effect of a remote origin for the information. Locating the apparent origin of the display image in a position far in front of the aircraft, where the pilot's eyes are normally focused during aircraft operation, eliminates the need to look down and refocus onto the aircraft instrument panel. Thus, the pilot may retain a constant head-up attitude.
Other information, such as fuel level or engine temperature may be displayed in the same manner.
Images such as those discussed here which have an apparent position where no real image source exists are known as "virtual images". Images generated in this manner can range in complexity from simple cursors or cross hairs, to complete scenery with trees and mountains. The types of images which may be used for simulation or vehicle operation in helmet mounted head-up displays are diverse in nature.
In a helmet mounted head-up display the image source is generally located in or near a helmet worn by the user. Special optics, also located in or near the helmet, bend and focus light from the image source to create a virtual image of the display. Helmet mounting has the advantage of allowing the image source to move with the user. Also, a head-mounted display generally serves to present a larger image field, in part because the display and optics are closer to the user's eye.
FIG. 1 illustrates a helmet mounted head-up display 10 which may be equipped with a stereo image source and controller 12. The controller and image source 12 supplies imagery to one or both eyes through a fiber optic bundle (FOB) 14 which contains an array of individual fibers which transmit light to produce the display to the wearer 16 of the helmet 18 as viewed from the eyes of the wearer 20 of the helmet. A relay lens 22 couples the image from the FOB 14 to the eyes. In applications using virtual imagery, computer generated images are transmitted by the FOB 14 from the controller and image source 12. The virtual image is spatially overlayed onto normal real world scenery outside the helmet 18. Furthermore, it is possible to block out the real world scenery and allow the wearer of the helmet mounted head-up display to only view the virtual scenery generated by the computer.
Certain systems utilizing a helmet mounted head-up display detect the position or angle of the helmet. This information is used by the controller and image source to adjust the virtual scenery presented to the user with the goal being to shift the scenery in a realistic manner as the helmet moves.
Applications of helmet mounted head-up displays are only effective if the virtual imagery presented to the wearer of the helmet is realistic. Realism depends strongly on two factors which are the resolution or detail of the virtual image and the apparent size of the imagery. Resolution is normally measured by the size of the smallest point (usually called a "pel" or "pixel") which can be displayed. Higher resolution imagery features smaller pixels and appears more precise and contains more detail. The image size is a measure of the vertical and horizontal size of the virtual image as it appears to the viewer, and can be measured in angular degrees or radians. A larger image reduces tunnel vision and more closely duplicates normal viewing. In most instances, the resolution and size of a helmet mounted head-up display are limited by the resolution of the device used as the image source. Standard image sources are usually capable of presenting a fixed, limited number of image pixels. These pixels can be used to create a small, highly resolved image, or can be spread out to make a larger, but less detailed image. In either case, the number of pixels is generally fixed. It is usually the task of the designer of a helmet mounted head-up display to adjust the resolution and size so as to create the most favorable image for a particular application.
The limit on the number of pixels which may be used is due to basic limitations in the present display technology. For the presently favored CRT image source, the number of pixels which can be presented by the display is roughly calculated by dividing the area of the display by the area of the smallest pixel which can be presented. This calculation yields the number of pixels which can be packed into the display area. This number can be increased either by increasing the display area or by reducing the size of the pixels. Both of these practices have reached practical limits in present technology. The display area cannot be increased because the display size and weight become cumbersome for the head of the user of a helmet mounted head-up display to support. Basic CRT electron gun and phosphor technology limits further significant decreases in the size of the display pixels.
As a practical example, a typical CRT used in helmet mounted head-up displays may be one inch in diameter, 4.5 inches long and weigh about 6 ounces. The pixel size is typically about 0.001 inch and a complete display image may contain approximately 650,000 pixels. The pixels are nominally arranged as adjacent lines on the CRT faceplate. Image resolution is traditionally identified as the number of lines and the number of pixels-per-line which the display can support. Thus, a display resolution might be described as 875 lines by 875 pixels-per-line. State of the art helmet mounted displays can produce images of 1,500 lines at 1,500 pixels-per-line for a total of more than 2,000,000 pixels. As mentioned above, display performance can also be rated according to the maximum number of pixels which can be displayed. This number is often referred to as the "space-bandwidth product", and is the product of the number of display lines times the number of pixels-per-line.
Thus, the objective of greater image realism is effectively limited by the inability of the head to support larger, heavier displays. In response, designers of helmet mounted head-up displays have removed the CRT image source from the helmet area and used a coherent FOB as illustrated in FIG. 1 to convey remotely generated images to the helmet region. A coherent FOB preserves a fixed address between input and output pixels such that a first pixel of an image source coupled to a first fiber is coupled by the fiber to a first pixel at a corresponding position in an image display. The spatial order of the image pixels is thus maintained during transmission. When the CRT is remotely located off the helmet, the CRT size can be increased to produce more highly resolved and sometimes brighter images which serve to increase the realism of the image.
The fiber optic bundle is constructed as an array of glass or plastic fibers which can each convey one unit of image information from the image source (CRT) to the helmet for display to the wearer of the helmet. To work effectively, the FOB must include a number of fibers equal to or exceeding the number of CRT pixels. Thus, in high resolution applications, the number of fibers in the bundle may exceed 2,000,000 (1,500 lines.times.1,500 pixels/line image source). In this case, if each individual fiber is 50 microns in diameter, the fiber bundle will be almost two inches in diameter. Due to the large number of individual fibers, such a bundle is almost impossible to manufacture without defects, is expensive, and generally represents a greater head weight than the head-mounted CRT it replaces. Weight considerations are particularly important in applications where high accelerations can radically increase the effective weight of the bundle and helmet. Such applications include helmet-mounted displays used in high performance aircraft. For these reasons, the approach of remotely locating the image source by using a fiber optic bundle has limited utility in high resolution applications.
Thus, current state of the art helmet mounted head-up displays demonstrate limited realism due to a limited image source space bandwidth product. Although designers realize the benefits of a higher resolution and larger imagery in helmet mounted head-up displays, no present technology is available to meet this need when considered from the perspective of the expense of manufacturing, reliability and weight of the necessary fiber optic bundle connecting a remote image source to the helmet of the helmet mounted head-up display especially for applications where high acceleration induced forces are expected, such as in high performance aircraft, or where mobility is required.
U.S. Pat. 4,897,715 discloses a helmet display in which a 1.times.n array of fiber optic fibers is deflected across the width of the viewing field to produce a stereoscopic image within the field of view (FOV) of the wearer of the helmet. The system of the '715 patent relies upon each fiber in the 1.times.n array scanning a plurality of lines within a predetermined area of the display as viewed from the FOV with the stated example being 125 lines. This system suffers from the disadvantage of requiring the deflection of the fiber optic ribbon across the total width of the display FOV, which presents substantial problems in the control of the motion sweeping the image produced by the 1.times.n array of fibers across the width of the image FOV and further adds to the expense of the system. For example, the '715 patent discloses that the resultant image has dimensions by 11.2 mm in the vertical direction and 15.4 mm in the horizontal direction. As a result, the deflection system for sweeping the 1.times.n array through the width of the display must cause a physical displacement of the light outputted by the array of fibers through a physical dimension which produces the width of 15.4 mm at the FOV. This large amplitude of motion presents difficult control problems which add expense to the display.
Image pick-up systems are known using image pickup devices such as charge coupled devices (CCDs) which either physically move the CCD array or optically move the optical image coupled to the CCD array to increase the resolution of the sensed pixels. These systems permit the production of a higher resolution video image beyond the resolution of individual CCD fabrication technology by moving the array and/or light coupled to the array. U.S. Pat. Nos. 4,543,601, 4,595,954, 4,612,581 and 4,652,928 disclose image sensing devices in which image sensing elements are physically moved relative to the object being scanned to provide a higher resolution sensing of pixels in the image than the physical resolution of the image sensing elements in the array which is used. Furthermore, U.S. Pat. Nos. 4,633,317, 6,755,876, 4,910,413 and 4,920,418 disclose image sensing devices in which the optical image is moved relative to the image sensing array to provide higher resolution of the sensed image than the resolution of the individual sensing elements within the array.
None of these patents discloses maintaining synchronism between the image sensing device and an image display with the image display producing a high resolution output by synchronizing the generation of successive fields of the image with the production of relative motion between an array of light emitting points by an image display as viewed from a FOV through successive positions which are synchronized with the generation of successive fields by the image sensing device.
U.S. Pat. Nos. 4,311,999 and 4,831,370 disclose displays in which a linear array of optical fibers is physically scanned across the width of scanning lines. These systems are analogous to the scanning produced by U.S. Pat. No. 4,897,715 described above.
U.S. Pat. No. 4,934,773 discloses a display in which a linear array of light emitting diodes (LEDs) is optically scanned across the entire width of the display.
U.S. Pat. No. 1,992,099 discloses a display device in which a disk containing a plurality of lenses is rotated by a motor to scan light from three light sources to produce a composite display.